1. Field of the Invention
The present invention relates to an optical fiber suitable for the optical transmission line in a wavelength-division-multiplexing (WDM) transmission system.
2. Related Background Arts
WDM transmission systems can transmit a large volumes of information at a high rate by using signal light having a multitude of wavelengths in a 1.55 xcexcm wavelength band. In a WDM transmission system, it is essential to increase the absolute value of the chromatic dispersion of the optical transmission line for suppressing the generation of nonlinear optical phenomena, especially four-wave mixing, and to decrease the absolute value of the dispersion slope of the optical transmission line for widening the bandwidth of wavelength of signal light.
An example of optical fibers for the optical transmission line in a WDM transmission system is a single-mode optical fiber stipulated in THE ITU G 652 Standard. The single-mode optical fiber has a chromatic dispersion as comparatively large as about 17 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm, so that it can suppress the generation of four-wave mixing. The optical fiber, however, has a dispersion slope as comparatively large as about 0.056 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm, so that it has no wide bandwidth of wavelength of signal light.
As other examples, the U.S. Pat. No. 5,684,909 and the published European Patent Application EP0883 002A1 disclose dispersion-flattened optical fibers that have a reduced dispersion slope at a wavelength of 1.55 xcexcm. The dispersion-flattened optical fibers have a desirable dispersion slope as small as not more than 0.05 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm. The optical fibers, however, have a chromatic dispersion as small as about 1 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm, so that they cannot sufficiently suppress the generation of four-wave mixing.
An object of the present invention is to offer an optical fiber that can sufficiently suppress the generation of four-wave mixing and that can widen the bandwidth of wavelength of signal light.
In order to achieve this object and other objects as well, the present invention offers an optical fiber that has a chromatic dispersion whose absolute value is not less than 8 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 and not more than 15 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm and that has a dispersion slope whose absolute value is not more than 0.05 psxc2x7nmxe2x88x922xc2x7kmxe2x88x921 at a wavelength of 1.55 xcexcm.
In an embodiment, the optical fiber of the present invention comprises the following regions:
(a) a first core region that includes the optical central axis and that has a first refractive index;
(b) a second core region that surrounds the first core region and that has a second refractive index that is smaller than the first refractive index;
(c) a third core region that surrounds the second core region and that has a third refractive index that is larger than the second refractive index; and
(d) a cladding region that surrounds the third core region and that has a fourth refractive index that is smaller than the third refractive index.
The description below, together with the accompanying drawings, gives a detailed explanation of the foregoing objects and the novel features of the present invention. It is to be understood that the accompanying drawings are provided for illustrating the present invention rather than for limiting the scope of application of the present invention.